Many people use mobile nodes, such as cell phones and personal digital assistants (PDAs), to communicate with and through wireless-communication networks. These mobile nodes and networks typically communicate over a radio-frequency (RF) air interface according to a wireless protocol such as Code Division Multiple Access (CDMA), perhaps in conformance with one or more industry specifications such as IS-95, IS-856, and IS-2000. Other protocols may be used as well, such as iDEN, TDMA, AMPS, GSM, GPRS, UMTS, EDGE, WiMAX (e.g., IEEE 802.16), LTE, microwave, satellite, MMDS, Wi-Fi (e.g., IEEE 802.11), and others now known or later developed.
In a typical wireless-communication network, a plurality of base stations are included, each of which defines one or more wireless coverage areas. A mobile node positioned in one of these wireless coverage areas can communicate over the RF air interface with the base station, which may provide the mobile node access to one or more circuit-switched, packet-switched, and/or other transport networks.
Mobile nodes and base stations conduct communication sessions (e.g. voice calls and data sessions) over frequencies known as carriers. A base station may provide service in a given wireless coverage area on one or more carriers. The communication from the wireless-communication network to a given mobile node are said to occur on a forward link, while those from a given mobile node to the wireless-communication network are said to occur on a reverse link. A typical mobile node periodically measures the strength of the carrier to determine a signal strength of the forward link.
A mobile node may travel between wireless coverage areas; for example, the mobile node may begin a wireless communication session in a first wireless coverage area and subsequently leave the first wireless coverage area. While the mobile node is being served in the first wireless coverage area, the mobile node may be served on a first carrier. As the mobile node leaves the first wireless coverage area, the mobile node may determine that the signal strength of the first carrier is relatively low.
Upon determining the signal strength of the first carrier is relatively low, the mobile node may request that the wireless-communication network perform a “handoff”; that is, a transfer of service between from the first carrier to a second carrier during a communication session. In particular, a “hard handoff” involves transferring the mobile node from the first carrier before establishing service on the second carrier while maintaining the communication session. In contrast, a “soft handoff” transfers the mobile node from the first carrier after establishing service on the second carrier while maintaining the communication session. After the handoff is performed, the mobile node is served on the second carrier.